Petrogenesis of Aoyougou high-silica adakite in the North Qilian orogen, NW China: Evidence for decompression melting of oceanic slab.

A trondhjemitic body occurs in the Aoyougou area, the western part of the North Qilian orogen. It is geochemically characterized by high SiO, NaO (high Na/K), elevated Sr/Y and (La/Yb)N, positive Sr anomaly, relatively enriched large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depleted Nb, Ta, Ti, resembling the high-silica adakite. Zircon U-Pb SHRIMP dating yields a weighted mean age of 438±3 Ma. This age is significantly younger than eclogitization ages of 460-490 Ma in the North Qilian orogen, suggesting that formation of the adakite postdates the subduction of oceanic crust in association with closure of the ancient Qilian Ocean. Whole-rock Sr and Nd isotopic analyses give initial ratios of I=0.7044-0.7047 and ɛ( t)=3.0-4.1, indicating that they are derived from partial melting of the juvenile oceanic crust. In view of the tectonic evolution of the North Qilian orogen, the high-silica adakite was probably derived from decompression melting of the exhumed eclogite at the depth of ∼60 km. [ABSTRACT FROM AUTHOR]